Metal tube lead-in bushing



Jan. 25, 1938. rrc 2,106,394

METAL TUBE LEAD-IN BUSHING Filed Nov. 25, 1936 INVENTOR LCRlS E MITCHELL ATTORNEY Patented Jan. 25, 1938 UNITED STATES PATENT QFFEQE mesne assignments,

to Radio Corporation of America, a corporation of Delaware Application November 25, 1936, Serial No. 112,635

3 Claims.

My invention relates to radio receiving tubes with metal envelopes and is particularly directed to the construction of the lead-in bushing at the top of the envelope.

In some types of all-metal tubes one of the external connections is made through an insulating bushing at the top of the envelope remote from the base. Usually the contactor or top cap to which external circuits are connected is supported on an insulator which is mechanically separate from the lead-in conductor in the top wall of the envelope.

According to my invention a simple unitary structure is provided for sealing the usual. lead-in conductor gas-tight in the envelope as well as for supporting the external circuit contactor. The

lead-in assembly constructed according to my invention is electrically eiiicient, mechanically strong, rugged in construction, simple in manu- 20 facture and neat in appearance.

The characteristic features of my invention are specifically defined in the appended claims and preferred embodiments thereof are disclosed in the following specification and accompanyingv drawing in which,

Figure 1 shows a sectioned perspective view of my improved lead-in assembly with the sealed lead-in and top cap in assembled position,

Figures 2 and 3 show sectional views of slightly modified forms of 1ead-in assemblies embodying my invention.

Referring to Figure 1 of the drawing, l designates an inverted cup-shaped metal envelope en closing any desired arrangement of electrodes. One of the electrodes, a grid by way of example, is shown with its lead 2 extending through the upper end of the envelope. In applying my invention to this type of tube I prefer to insulate and support the grid lead centrally in an opening 3 in the upper end of the envelope by a mass of insulating material 4, such as glass, which is sealed gas-tight to the inner edge of a metal annulus or washer 5 of a metal with a coeificient of expansion such that it makes a good seal with the glass. Annulus 5 is conveniently sealed concentrically in the opening of the envelope by welding or brazing, welding being facilitated by stamping the Washer with a ridge 6 along its periphery which it has been found offers sufficient resistance to the flow of welding current between the washer and the shell to heat and fuse the two metal parts together. A second ridge 6a. concentric with and inside the first ridge may be stamped in the washer to center the washer in the opening of the envelope.

In accordance with my invention the mass of glass 4 projects from the washer 5 sufficiently to constitute an insulating support for the top capor contactor which is mechanically supported and also electrically insulated from the washer by the mass of giass. The glass-to-metal seal thus made admirably lends itself to the attacl1- merit and rigid support of the contactor l, which comprises essentially a small thimble of thin drawn metal, such as nickel or nickel plated brass, with a punched out opening at its upper end to receive the outer end of the lead-in conductor. The thimble is lowered over the projecting mass of glass i until it is in proper position, and is then cemented to the glass by the usual basing cement. The thirnble may, if desired, be

heated and adhesively fused to the glass. The conductor is then trimmed off at the top of the thimble and soldered to it.

To prevent the clamp or connector for an external circuit connected to the thimble from slipping downwardly over the thimble and into contact with the metal envelope, it has been found convenient to form an annular shoulder intermediate its ends as shown at 8.

To increase the bearing surface between the glass and the metal washer to which it is sealed, the washer may conveniently be sealed over the opening in the envelope as shown in Figure 2. Ring 5 is preferably stamped with a concentric ridge to facilitate welding, as described in connection with Figure l. The ridge, however, may be flattened against the top wall. of the envelope by heat and pressure created between welding electrodes during welding to produce a flat gas tight junction of large area between the washer and envelope. In such a construction it is desirable that the flow of the molten glass be limited at the outer edge of the seal by a turned-up flange 9 on the rim of the Washer. The sealing of the lead-in wire and the attachment of the top cap is the same as described in connection with Figure 1.

An alternative arrangement for limiting the outward flow of the molten glass is shown in Fig ure 3, where the edge of the opening 3 in the envelope is turned up in a shallow flange iii. In manufacture, this construction is particularly uniform. and neat in appearance.

In manufacturing my novel top cap assembly, I have obtained good results by first welding over the opening in the top of a drawn iron envelope a washer .70 inch in diameter and .025 inch thick of chrome-iron alloy, commerically known as Allegheny metal. An electrode assembly mounted upon a header and with the usual leadin conductor 2 is then inserted in the envelope with the conductor extending centrally through the Washer. A short length of tubing of commercial lime glass commonly used in the art is slipped over the lead-in conductor adjacent the opening of the Washer whereupon gas flames are directed on the glass to raise it to such a temperature that the glass becomes plastic and flows into sealing engagement with the conductor and with the washer as shown. molten glass may be pressed with the plunger of a molding form to speed the sealing operation and set the glass in the desired shape, the inner ridge Ea, Figure 1, conveniently limiting the radial flow of the glass. Alternatively, the washer 5 may be of an iron nickel cobalt alloy commercially known as Fernico, the glass of the hard variety commercially known as 702? glas and the conductor of some metal, such as tungsten or molybdenum, to which hard glass will seal.

A metal radio receiving tube constructed according to this invention is inexpensive to manufacture, is electrically efficient, is mechanically strong and is particularly neat in appearance. It will be obvious to those skilled in the art that various modifications may be made in my device without departing from the spirit thereof. It is accordingly desired that my invention be limited only by the prior art and by the accompanying claims.

I claim:

1. A radio receiving tube comprising a metal envelope, an opening in the wall of said envelope, a metal washer concentric with and sealed gastight to the envelope over said opening, a lead-in conductor extending centrally through the opening of said washer and insulatingly sealed gastight to said Washer by a mass of glass of a temp rature co-eificient of expansion substantially that of said washer, and a thimble-like contactor electrically connected to said conductor and mechanically supported on and insulated from said envelope by said mass of glass.

2. An electron discharge device comprising a metal envelope having a perforation, a lead-in conductor extending centrally through said perforation and insulatingly sealed gas-tight in said envelope with a mass of glass projected externally from the envelope, and a contactor thimble mechanically supported by and adhesively attached to the outwardly projecting portion of said glass and electrically connected to the conductor.

3. An electron discharge device comprising a thimble-like contactor as defined in claim 2 with a shoulder intermediate its ends.

LORIS E. MITCHELL. 

